LED with current confinement structure and surface roughening

ABSTRACT

A light emitting diode having a vertical orientation with an ohmic contact on portions of a top surface of the diode and a reflective layer adjacent the light emitting region of the diode. This light emitting diode includes a confinement structure. The confinement structure may be an opening in the reflective layer generally beneath the top ohmic contact that defines a non-contact area between the reflective layer and the light emitting region of the diode to encourage current flow to take place other than at the non-contact area to in turn decrease the number of light emitting recombinations beneath the ohmic contact and increase the number of light emitting recombinations in the areas not beneath said ohmic contact. The LED may include roughened emitting surfaces to further enhance light extraction.

This application is a continuation application from, and claims thebenefit of, U.S. patent application Ser. No. 11/983,515, filed on Nov.9, 2007, now U.S. Pat. No. 8,410,490 which is a divisional applicationfrom, and claims the benefit of, U.S. patent application Ser. No.11/042,030, filed on Jan. 24, 2005, and now issued as U.S. Pat. No.7,335,920.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to light emitting diodes (LEDs) and moreparticularly to new structures for enhancing the extraction of lightfrom LEDs.

2. Description of Related Art

Light emitting diodes (LEDs) are an important class of solid statedevices that convert electric energy to light and generally comprise anactive layer of semiconductor material sandwiched between two oppositelydoped layers. When a bias is applied across the doped layers, holes andelectrons are injected into the active layer where they recombine togenerate light. Light is emitted omnidirectionally from the active layerand from all surfaces of the LED.

There has been a great deal of recent interest in LEDs formed ofGroup-III nitride based material systems because of their uniquecombination of material characteristics including high breakdown fields,wide bandgaps (3.36 eV for GaN at room temperature), large conductionband offset, and high saturated electron drift velocity. The doped andactive layers are typically formed on a substrate that can be made ofdifferent materials such as silicon (Si), silicon carbide (SiC), andsapphire (Al₂O₃). SiC wafers are often preferred because they have amuch closer crystal lattice match to Group-III nitrides, which resultsin Group III nitride films of higher quality. SiC also has a very highthermal conductivity so that the total output power of Group III nitridedevices on SiC is not limited by the thermal resistance of the wafer (asis the case with some devices formed on sapphire or Si). Also, theavailability of semi insulating SiC wafers provides the capacity fordevice isolation and reduced parasitic capacitance that make commercialdevices possible. SiC substrates are available from Cree Inc., ofDurham, N.C. and methods for producing them are set forth in thescientific literature as well as in U.S. Pat. No. Re. 34,861; U.S. Pat.No. 4,946,547; and U.S. Pat. No. 5,200,022.

The efficient extraction of light from LEDs is a major concern in thefabrication of high efficiency LEDs. For conventional LEDs with a singleout-coupling surface, the external quantum efficiency is limited bytotal internal reflection (TIR) of light from the LED's emission regionthat passes through the substrate. TIR can be caused by the largedifference in the refractive index between the LED's semiconductor andsurrounding ambient. LEDs with SiC substrates have relatively low lightextraction efficiencies because the high index of refraction of SiC(approximately 2.7) compared to the index of refraction for thesurrounding material, such as epoxy (approximately 1.5). This differenceresults in a small escape cone from which light rays from the activearea can transmit from the SiC substrate into the epoxy and ultimatelyescape from the LED package.

Different approaches have been developed to reduce TIR and improveoverall light extraction, with one of the more popular being surfacetexturing. Surface texturing increases the light's escape probability byproviding a varying surface that allows photons multiple opportunitiesto find an escape cone. Light that does not find an escape conecontinues to experience TIR, and reflects off the textured surface atdifferent angles until it finds an escape cone. The benefits of surfacetexturing have been discussed in several articles. [See Windisch et al.,Impact of Texture-Enhanced Transmission on High-Efficiency SurfaceTextured Light Emitting Diodes, Appl. Phys. Lett., Vol. 79, No. 15,October 2001, Pgs. 2316-2317; Schnitzer et al. 30% External QuantumEfficiency From Surface Textured, Thin Film Light Emitting Diodes, Appl.Phys. Lett., Vol 64, No. 16, October 1993, Pgs. 2174-2176; Windisch etal. Light Extraction Mechanisms in High-Efficiency Surface TexturedLight Emitting Diodes, IEEE Journal on Selected Topics in QuantumElectronics, Vol. 8, No. 2, March/April 2002, Pgs. 248-255; Streubel etal. High Brightness AlGaNInP Light Emitting Diodes, IEEE Journal onSelected Topics in Quantum Electronics, Vol. 8, No. March/April 2002].

U.S. Pat. No. 6,410,942, assigned to Cree Inc., discloses an LEDstructure that includes an array of electrically interconnected microLEDs formed between first and second spreading layers. When a bias isapplied across the spreaders, the micro LEDs emit light. Light from eachof the micro LEDs reaches a surface after traveling only a shortdistance, thereby reducing TIR.

U.S. Pat. No. 6,657,236, also assigned to Cree Inc., disclosesstructures for enhancing light extraction in LEDs through the use ofinternal and external optical elements formed in an array. The opticalelements have many different shapes, such as hemispheres and pyramids,and may be located on the surface of, or within, various layers of theLED. The elements provide surfaces from which light refracts orscatters.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the invention is directed to LEDs havingenhanced light extraction features. In one aspect of the invention, theLED includes a p-type layer of material with an associated p-contact, ann-type layer of material with an associated n-contact and an activeregion between the p-type layer and the n-type layer. The LED furtherincludes a confinement structure that is formed within at least one ofthe p-type layer of material and the n-type layer of material. Theconfinement structure substantially prevents the emission of light fromthe area of the active region that is coincident with the area of theconfinement structure. The LED also includes a roughened surface that isassociated with one of the p-type and n-type layers of material.

In another aspect of the invention, the LED includes a first layer ofmaterial with an associated first contact and first surface throughwhich light is emitted, a second layer of material with an associatedsecond contact and an active region between the first layer and thesecond layer. The LED further includes a confinement structure that isintegral with one of the first layer and the second layer. Theconfinement structure is generally axially aligned with the firstcontact and substantially prevents the emission of light in the area ofthe active region that is coincident with the area of the confinementstructure.

In yet another aspect of the invention, the LED includes a first layerof material with an associated first contact and first surface throughwhich light is emitted, a second layer of material, an active regionbetween the first layer and the second layer, and a conducting substrateadjacent the second layer of material that has an associated substratecontact. The LED further includes at least one confinement structurethat is within one of the first layer, the second layer and thesubstrate. The confinement structure is generally axially aligned withthe first contact and directs current flowing toward the active regionaway from the area of the active region that is coincident with the areaof the confinement structure.

In still another aspect of the invention, the LED includes a first layerof material with an associated first contact and first surface throughwhich light is emitted, a second layer of material with an associatedsecond contact and an active region between the first layer and thesecond layer. The LED further includes a confinement structure that isassociated with the second contact. The confinement structure directscurrent flowing toward the active region away from the area of theactive region that is coincident with the area of the confinementstructure.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of one general embodiment of an LED includingan active region between two layers of conducting material and a currentconfinement structure that may be located in either layer of theconducting material;

FIG. 2 is a cross section of another general embodiment of an LEDincluding an active region between two layers of conducting material, asubstrate, a roughened top surface and a current confinement structurethat may be located in either layer of the conducting material or asubstrate;

FIG. 3 is a cross section of a configuration of the LED of FIG. 1,including a current confinement structure in a bottom layer of n-typematerial;

FIG. 4 is a cross section of a configuration of the LED of FIG. 2,including a current confinement structure in a top layer of p-typematerial and a layer of transparent conducting material having aroughened top surface;

FIG. 5 is a cross section of a configuration of the LED of FIG. 1,including a current confinement structure in a bottom layer of p-typematerial and a layer of n-type material having a roughened top surface;

FIG. 6 is a cross section of a configuration of the LED of FIG. 1,including a current confinement structure in a top layer of n-typematerial and a layer of n-type material having a roughened top surface;and

FIG. 7 is a cross section of another general embodiment of an LEDincluding an active region between two layers of conducting material, atop side contact, a bottom side contact and a current confinementstructure located in the layer of the bottom side contact.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides improved light extraction for lightemitting diodes (LEDs) through a confinement structure that is formedwithin at least one of the p-type layer of material and the n-type layerof material of a base LED structure. The confinement structure isgenerally aligned with the contact on the top and primary emissionsurface of the LED and substantially prevents the emission of light fromthe area of the active region that is coincident with the area of theconfinement structure and the top-surface contact. Thus, light thatwould otherwise emit under and be absorbed by the top-surface contact isredirected to other regions of the active layer and the emitting sidewhere the absorbing affect of the contact is substantially reduced. In apreferred embodiment, the current confinement structure is formed withinthe based LED structure using ion implantation. The current confinementstructure may also be formed in the LED base structure using selectiveoxidation. The current confinement structure may also be formed as partof the LED structure using epitaxial regrowth.

The LED may further include a roughed surface around the absorbingcontact. The roughened surface may be included in all or a portion ofthe surface area of a layer of the base LED structure or in all or aportion of the surface area of an additional layer of material appliedto the base LED structure. For example, in an n-side up LED structurehaving a sufficiently thick n-type layer of material it may bepreferable to roughen the n-type layer. In a p-side up base LEDstructure having a relatively thin layer of p-type material, it may bepreferable to add a layer of transparent material to the p-type layerand roughen that layer. A layer of transparent material may also beadded to the n-type layer of an n-side up LED structure. In either case,the combination of a roughened surface and current confinement structurethat directs current toward the roughened surface and away from theabsorbing contact provides further enhanced light extraction. Theroughened surface improves light extraction by providing a varyingsurface that allows light that would otherwise be trapped in the LED bytotal internal reflection to escape and contribute to light emission.

Referring now to the drawings and particularly to FIGS. 1 and 2, thereis shown one embodiment of an LED 10 according to the present invention,including a first layer of material 12 having a first surface 14 throughwhich light is emitted, a second layer of material 16, and a layer ofactive material 18 sandwiched between the first layer and the secondlayer. The first layer 12, second layer 16 and active layer 18 form abase LED structure that is positioned on a support structure 36.

The base LED structure may be fabricated from different semiconductormaterial systems such as the Group III nitride based material systems.Group III nitrides refer to those semiconductor compounds formed betweennitrogen and the elements in the Group III of the periodic table,usually aluminum (Al), gallium (Ga), and indium (In). The term alsorefers to ternary and tertiary compounds such as AlGaN and AlInGaN. In apreferred embodiment, the layer of active material 18 is in adjacentcontact with the first layer 12 and the second layer 16, and thematerial forming the first and second layers is GaN, with either of thefirst or second layers being p-type material and the other layer beingn-type material. In this embodiment, the material forming the activelayer is InGaN. In alternative embodiments the first and second layermaterial may be AlGaN, AlGaAs or AlGaInP.

Depending on the LED configuration, the support structure 36 which maybe a substrate or a submount. In a p-side up LED configuration, thesupport structure 36 would be a substrate, with a suitable substratebeing a 4H polytype of silicon carbide, although other silicon carbidepolytypes can also be used including 3C, 6H and 15R polytypes. Siliconcarbide has a much closer crystal lattice match to Group III nitridesthan sapphire and results in Group III nitride films of higher quality.Silicon carbide also has a very high thermal conductivity so that thetotal output power of Group III nitride devices on silicon carbide isnot limited by the thermal dissipation of the substrate (as may be thecase with some devices formed on sapphire). Also, the availability ofsilicon carbide substrates provides the capacity for device isolationand reduced parasitic capacitance that make commercial devices possible.SiC substrates are available from Cree, Inc., of Durham, N.C. andmethods for producing them are set forth in the scientific literature aswell as in a U.S. Pat. No. Re. 34,861; U.S. Pat. No. 4,946,547; and U.S.Pat. No. 5,200,022. For an n-side up LED configuration, the supportstructure 36 would be a submount.

A first contact 22 is associated with the first layer 12 and a secondcontact 24 is associated with the second layer 16. The association ofthe contacts 22, 24 with their respective layers 12, 16 may be direct orindirect. A direct association, wherein the first contact is in directcontact with the first layer 12 and the second contact 24 is in directcontact with the second layer 16, is shown in FIG. 1. For the secondcontact 24, this association may be present when the substrate 36 isformed of a non-conducting material. Indirect associations are shown inFIG. 2 and may be present, with respect to the first contact 22, if theLED includes a layer of transparent conducting material 25, and for thesecond contact 24, if the support structure 36 is a substrate formed ofa conducting material. To enhance light extraction in both direct andindirect contacting, the second contact may be formed of a reflectivematerial, such as silver (Ag), aluminum (Al) or rhodium (Rh).

A current confinement structure 20 is integral with and can be formed indifferent location within the LED, such as within at least one of thefirst layer 12, the second layer 16 or the substrate 36 (as shown inFIG. 2). In some embodiments, more than one current confinementstructure may be used and in one embodiment the current confinementstructure 20 may be formed in both the first layer 12 and the secondlayer 16 with a portion of the active material 18 between theconfinement structures 20. In some embodiments, the confinementstructure 20 may be a region of the layer of material whose crystalstructure or molecular properties have been altered through processesknown in the art, such as ion implantation or oxidation. In otherembodiments, the confinement structure 20 may be a current blockinglayer formed from a material that is oppositely doped relative to thematerial of the first or second layers 12, 16. This current blockinglayer of material may be incorporated into one or more of the first andsecond layers through the known process of epitaxial regrowth.

The current confinement structure 20 is positioned relative to the firstcontact 22 such that the center 26 or axis of the first contact isgenerally aligned with the center or axis 28 of the confinementstructure. The cross-sectional area size of the confinement structure 20essentially mirrors that of the first contact 22. The thickness of thecurrent confinement structure 20 may range anywhere from between 0.1% to80% of the total thickness of the layer. For example, in an n-type layerof material 1 micron thick, the current confinement structure 20 may bebetween 0.001 and 0.8 microns thick.

The current confinement structure 20 directs current 30 that is flowingtoward the active region 18 away from the portion 32 of the activeregion that is substantially coincident with and aligned with the firstcontact 22. This redirection of current substantially prevents therecombination of current charges, i.e., “holes” and “electrons,” in theportion 32 of the active region aligned with the first contact 22, thusessentially rendering the region inactive.

Light 34 is emitted from the active material 18 and propagates throughthe LED structure. Although, light emits from the active material 18 inall directions, for ease of illustration, light in the figures is shownonly in the upward direction toward the top or primary emission surfaceof the LED. In FIG. 1, the top surface is the surface 14 of the toplayer of material 12. In FIG. 2, the top surface is a layer of roughenedmaterial 25.

With reference to FIG. 3, one embodiment of the general LED of FIG. 1according to the present invention is shown comprising a p-side up LEDthat includes a first layer 40 of p-type material and a second layer 42of n-type material. In a preferred embodiment, the material is GaN. Asexplained further below, during the LED manufacturing process, thecurrent confinement structure 44 is incorporated into the n-typematerial layer 42. The structure 44 is formed by introducing impuritiesinto the n-type material. The introduction of impurities may be done byion implantation. For example, for an n-type GaN material, either Al orGa ions may be implanted.

Upon the application of a bias across the p-contact 46 and the n-contact48, current (in the form of p-type material “hole” movement) movesthrough the p-type material toward and into the active region. Likewise,current (in the form of n-type material “electron” movement) movesthrough the n-type material 42 toward and into the active region 50. Dueto the impurities of the confinement structure 44, the current movingthrough the n-type material 42 moves away from the current confinementstructure and enters the active region 50 in the area 52 around theinactive portion 54 of the active region that is substantiallycoincident with and aligned with the current confinement structure. Thisarea of the active region is referred to as the active portion 52 of theactive region.

The current moving through the p-type material 40 also moves away fromthe current confinement structure 44 and into the areas of the activeregion where the current from the n-type material has entered. Thismovement of the p-type material current is a result of a combination ofboth the presence of the current confinement structure 44 in the n-typematerial and the attraction of the p-type current “holes” to the n-typecurrent “electrons” present in the active portion 52 of the activeregion 50.

The current confinement structure 44 may be positioned at any one ofseveral locations along the depth of the n-type layer 42. This may bedone by interrupting the growth process of the n-type layer 42,implanting the impurities into the incomplete n-type layer and thenresuming the growth process to complete the rest of the n-type layer.The growth process may be any one of various known processes includingmetal oxide chemical vapor deposition (MOCVD), hybrid vapor phaseepitaxy (HVPE) or molecular beam epitaxy (MBE). An exemplary confinementstructure formation process includes implanting the n-layer with 180 keValuminum ions in doses of 10¹³, 10¹⁴ and 10¹⁵ cm⁻². In a preferredembodiment, the current confinement structure 44 is located close toactive region 50 to effectively prevent n-type current from movingcompletely around the current confinement structure and back toward theinactive region 54, and to increase the affect of the structure on thecurrent in the p-type layer on the opposite side of the active region50.

With reference to FIG. 4, another embodiment of the general LED 10 ofFIG. 2 according to the present invention is shown comprising a p-sideup LED that is essentially the same as that described with reference toFIG. 3, except the current confinement structure 44 is located in thep-type layer 40. Also, the substrate 58 is conducting thus allowing foran indirect association between the n-contact 48 and the n-type layer42. A layer of transparent conducting material 56 is included on thep-type layer 40 with a portion of the conducting material layersandwiched between the p-contact 46 and the p-type layer. This layer ofmaterial may be formed from ZnO, In₂O₃ and indium tin oxide (ITO). Atleast part of the conducting material layer 56 not covered by thep-contact 46 is roughened, with all of the top surface of the conductingmaterial layer as shown in FIG. 4 being roughened. The combination ofthe layer of transparent conducting material 56 and localized lightgeneration away from the absorbing contact 46, provided by the currentconfinement structure 44, increases the light extraction efficiency ofthe LED.

With reference to FIG. 5, another embodiment of the general LED 10 ofFIG. 1 is shown, comprising an n-side up, flip-chip LED that includes afirst layer 60 of n-type material and a second layer 62 of p-typematerial. As an additional processing step, the substrate which istypically adjacent to the first n-type layer 60 is removed to reveal thetop primary emitting surface of the LED.

In a preferred embodiment, the LED material is GaN. During the LEDmanufacturing process, the current confinement structure 64 isincorporated into the p-type material layer 62 prior to flipping layers60, 62, 64 and p-contact substructure onto a submount 78. Theconfinement structure 64 is formed by introducing impurities into thep-type material by ion implantation during growth. For example, for ap-type GaN material, either Al or Ga ions may be implanted. The topsurface 66 of the n-type layer is roughened to form a roughened lightextraction surface. The roughened surface may be provided by etching,using any one of several methods known in the art, such asphotoelectrochemical (PEC) etching. In this configuration, the roughenedsurface is added directly to the n-type layer instead of through aseparately added layer of transparent conducting material, as istypically required for p-side up LEDs, due to the relative thinness ofthe p-layer.

Upon the application of a bias across the n-contact 68 and the p-contact70, current moves through the p-type material toward and into the activeregion 72. Likewise, current moves through the n-type material 60 towardand into the active region 72. The current moving through the p-typematerial 62 moves away from the current confinement structure 64 andenters the active region 72 in the active portion 74 around the inactiveportion 76. The current moving through the n-type material 60 also movesaway from the current confinement structure 64 and into the activeportion 74 of the active region.

As with the embodiment of FIG. 3, the current confinement structure 64may be positioned at various locations along the depth of the p-typelayer 62 by interrupting the growth process of the p-type layer,implanting the impurities into the incomplete p-type layer and thenresuming the growth process to complete the layer. In a preferredembodiment, the current confinement structure 64 is located close toactive region 72 to effectively prevent p-type current from movingcompletely around the current confinement structure and back toward theinactive region 76, and to increase the affect of the structure on thecurrent in the n-type layer on the opposite side of the active region72. With reference to FIG. 6, in another embodiment, the general LED ofFIG. 1 is an n-side up LED that is essentially the same as thatdescribed with reference to FIG. 5, except the current confinementstructure 64 is located in the n-type layer 60.

Referring now to FIG. 7 there is shown an LED 100 including a firstlayer of material 102 having a first surface 104 through which most ofthe light is emitted, a second layer of material 106 and a layer ofactive material 108 sandwiched between the first layer and the secondlayer. In a preferred embodiment, the layer of active material 108 is inadjacent contact with the first layer 102 and the second layer 106, andthe material forming the first and second layers is GaN and the materialforming the active layer is InGaN. In alternative embodiments the firstand second layer material may be AlGaN, AlGaAs or AlGaInP.

A first contact 110 is associated with the first layer 102 and a secondcontact 112 is associated with the second layer 106. A currentconfinement structure 114 is included in the layer of the second contact112 and is positioned relative to the first contact 110 such that thecenter 116 or axis of the first contact is generally aligned with thecenter or axis 118 of the confinement structure. The layer of the secondcontact 112 and the confinement structure 114 is formed by depositing alayer of contact material, etching away a portion of the layer ofcontact material and replacing it with the material of the confinementstructure. The confinement structure 114 is formed of an insulating,non-conducting material, such as SiO₂, AlN and SiN, and has across-sectional area size essentially the same as that of the firstcontact 110.

The current confinement structure 114 directs current 120 that isflowing toward the active region 108 away from the portion 122 of theactive region that is substantially coincident with and aligned with thefirst contact 110. This redirection of current substantially preventsthe recombination of current charges, i.e., “holes” and “electrons,” inthe portion 122 of the active region aligned with the first contact 110,thus essentially rendering the region inactive.

As with the embodiments described with reference to FIGS. 3 through 6,the general LED of FIG. 7 may be formed such that the first layer 102 iseither one of an n-type layer or a p-type layer and the second layer 106is a type of layer opposite that of the first layer. The LED 100 mayalso include a surface roughening, either in the form of a roughened topsurface of the first layer 102 or an additional layer of transparentconducting material having a roughened top surface.

It will be appreciated by those of skill in the art that the concepts ofthe invention, as described in the embodiments of FIGS. 1-7, may beincorporated into other LED configurations. For example, while the LEDsof FIGS. 1-7 have contacts in a vertical arrangement, i.e., on oppositesides of the LED, the invention may be applied to LEDs having laterallyarranged contacts, i.e., on the same side of the LED, such as resonantcavity LEDs.

It will be apparent from the foregoing that while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

We claim:
 1. A light emitting diode having a vertical orientation,comprising: a top ohmic contact on portions of a top surface of saiddiode; a reflective layer adjacent a semiconductor material layer ofsaid diode; a passivated portion embedded in, and completely surroundedby, said semiconductor material layer beneath said top ohmic contactthat defines a less conductive area between said reflective layer and anactive region of said diode; and a roughened surface of said lightemitting diode.
 2. The light emitting diode according to claim 1 on asubstrate and having an ohmic contact to said substrate that is largerin area than said top ohmic contact.
 3. The light emitting diodeaccording to claim 2, wherein said reflective layer is positionedbetween said substrate and said semiconductor material layer.
 4. Thelight emitting diode according to claim 1, wherein the size of saidpassivated portion is substantially the same area as the size of saidtop ohmic contact.
 5. The light emitting diode according to claim 1,wherein said semiconductor material layer comprises a plurality of GroupIII nitride layers, including at least one p-type layer and at least onen-type layer.
 6. The light emitting diode according to claim 1, whereinsaid passivation portion comprises an ion implanted region.
 7. The lightemitting diode according to claim 1, wherein said passivation portioncomprises an oxidation region.
 8. The light emitting diode according toclaim 1, wherein said passivation portion comprises a current blockingregion.
 9. The light emitting diode according to claim 1, wherein saidroughened surface comprises a roughened top surface of said lightemitting diode.
 10. The light emitting diode according to claim 9,wherein said roughened top surface comprises a roughened semiconductormaterial layer.
 11. The light emitting diode according to claim 9,further comprising a transparent conducting layer on said light emittingdiode and wherein said roughened top surface comprises a roughened topsurface of said transparent conducting layer.
 12. The light emittingdiode according to claim 1, wherein said semiconductor material layer isa unitary semiconductor material layer and wherein said passivatedportion is embedded in said unitary semiconductor layer.